Conventionally, various techniques have been proposed for analyzing a golf swing with the goal of performing swing evaluation and the like in order to perform golf goods development, select a golf club that suits a golfer (perform so-called fitting), and improve one's golf skill. In JP 2013-90862A, in order to achieve an improvement in golf swing, the force transferred to a golf club during golf swings is derived in order to discover a golf swing that realizes an efficient transfer of energy from the golfer's arm to the golf club. Also, in JP 2013-90862A, the force that acts at the center of a line that connects the golfer's shoulders is calculated in the process of deriving the force transferred to the golf club.
Incidentally, in golf, the ability to stably hit a ball in a straight line for a long distance leads to an improvement in score. Accordingly, there is significance in evaluating golf swing stability. However, conventionally, the causes of variation in golf swings have not been sufficiently understood, and an appropriate indicator that enables evaluating the stability of a golf swing has not been known. For this reason, in order to evaluate the stability of a golf swing, it has been necessary to, for example, actually collect data for a large number of golf swings, and then investigate whether the hitting of the balls is stable. However, in situations where the stability of a golf swing needs to be evaluated, such as when developing golf goods and performing golf club fitting, actually collecting data for a large number of golf swings is not only inefficient, but also sometimes unrealistic.